Need to change my brakefluid, manual says dot 5.1, however I only do road riding. Will putting in dot 4 do any harm?

No

You can use DOT 4 in a pinch but I'd stick with using DOT 5.1 as it is the recommended fluid for the brakes on the SuperDuke. I don't think I'd want to skimp out on brakes myself.

Or replace all the seals and hoses for a few buck saved and a trip to the bike shop.

That what it think, but I am no chemiest...fook..cant even spell it.

SDnerd....???? howz about it...

How would dot 4 mess up my brake components?

Doesn't dot 5.1 expire pretty quick?

Manual says DOT 5.1, I'd stick with it.
It's your brakes, is a few dollars saved worth it?

They're both Glycol-ether / organic based oils so wouldn't necessarily damage any components but performance will be very, very different. dot 4 has a much lower boiling point /viscosity which means it will fail (vaporize in the lines) much sooner than a dot 5.1. Your set up; brakes, hoses, master cylinder, rotors, pads, etc. need the right grade brake fluid.

Put in 4 if you're not going to drive like an old maid and only apply a little brake pressure. 5.1 if you like you're bike and your mobility

fook ... just like motor oil weights and brands, eh?

DOT4 will work perfectly fine.

I use a "DOT4" fluid, which also happens to exceed the performance requirements of not only 5.1, but also of any 5.1 product available. The only real difference (other than marketing hyperbole), is the requirement of a somewhat higher boiling temperature. Unless you're racing the bike, and then only on courses where extreme changes in velocity take place several times per lap, it won't matter. On the street, it will never make a difference.

Glycol-based brake fluid is not an oil BTW - they are NOT petroleum-based.

Probably stated elsewhere, but please don't confuse DOT 5.1 with 5. 5 is silicone based, and is not compatible with systems designed to use glycol brake fluids.

Dot 5.1 is a lower viscosity brake fluid than Dot 3 /Dot 4 / Super Dot 4,
The dry / wet boiling point of Dot 5.1 is similar to Dot 4 but lower than Super Dot 4,
Thats it